One difficulty in fabricating nanostructural devices arises in the context of generating large-scale repeating patterns of nanostructures. Difficulties arise in controlling the spatial locations of the nanostructures, controlling the uniformity of the nanostructures, and/or varying the individual characteristics of the nanostructures in a controlled manner. The ability to rely on photolithographic techniques diminishes substantially as the target structure size falls below about 500 nm and, despite recent advancements in extreme ultraviolet lithography and x-ray lithography, becomes extremely difficult and costly below about 100 nm. Although alternative writing technologies including electron beam lithography and ion beam lithography might provide for adequate control of the nanostructural characteristics, these techniques have limited speed and limited scalability that would reduce their practicality in a mass production setting. The above difficulties increase further if the desired structures are three-dimensional in character, i.e., have an appreciable elevation dimension in addition to a two-dimensional footprint characteristic.
One use for devices comprising large-scale repeating patterns of nanostructures, particularly metallic nanostructures, relates to controlling the propagation of electromagnetic radiation in the infrared, near infrared, visible, and/or ultraviolet frequency ranges. Substantial attention has been directed in recent years toward composite materials capable of exhibiting negative effective permeability and/or negative effective permittivity with respect to incident electromagnetic radiation. Such materials, often interchangeably termed artificial materials or metamaterials, generally comprise periodic arrays of electromagnetically resonant cells that are of substantially small dimension (e.g., 20% or less) compared to the wavelength of the incident radiation. Although the individual response of any particular cell to an incident wavefront can be quite complicated, the aggregate response the resonant cells can be described macroscopically, as if the composite material were a continuous material, except that the permeability term is replaced by an effective permeability and the permittivity term is replaced by an effective permittivity. However, unlike continuous materials, the structure of resonant cells can be manipulated to vary their magnetic and electrical properties, such that different ranges of effective permeability and/or effective permittivity can be achieved across various useful radiation wavelengths.
Of particular appeal are so-called negative index materials, sometimes referred to as left-handed materials, in which the effective permeability and effective permittivity are simultaneously negative for one or more wavelengths depending on the size, structure, and arrangement of the resonant cells. Potential industrial applicabilities for negative-index materials include so-called superlenses having the ability to image far below the diffraction limit to λ/6 and beyond.
One type of composite material known to exhibit negative effective permeability and/or negative effective permittivity comprises a uniform periodic array of conducting metallic structures. Each metallic structure resembles a tube or hollowed-out cylinder and is formed around a core region, the core region comprising air or other substantially non-conducting material. However, due at least in part to the fabrication difficulties described above, known implementations of such arrays have comprised relatively large-sized elements directed to controlling electromagnetic radiation at or near microwave frequencies, much larger than the element sizes required for controlling infrared, near infrared, visible, and/or ultraviolet radiation.
Accordingly, it would be desirable to provide a method for fabricating a patterned array of metallic nanostructures in a manner that allows for large-scale arrays thereof to be constructed with uniform, or otherwise carefully controlled, physical and electrical characteristics.
It would be further desirable for such fabrication method to provide for individual metallic nanostructural dimensions that are substantially less than the wavelength of infrared, near infrared, visible, and/or ultraviolet light.
It would be still further desirable for such fabrication method to be highly scalable for implementation in a mass production environment.
It would be even further desirable to provide a composite material designed to exhibit at least one of a negative effective permeability and negative effective permittivity for incident radiation of at least one infrared, near infrared, visible, or ultraviolet frequency constructed according to such fabrication process.